As one of safety technologies for reducing traffic accidents, technologies for improving the driving stability have been developing. The technologies for improving the handling and stability are the yaw-moment control method by transverse distribution of driving/braking forces, the front wheel active steering control method, etc. Various proposals have been made for the steering gear ratio control technique in a front wheel active steering system where the front wheels of a vehicle are not mechanically connected to the steering wheel. By the steering gear ratio control technique the gear ratio (steering angle ratio) of the steering wheel ratio to the actual steering angle of the wheel tires is changed according to the running condition. As a result, the vehicle response can be matched with the driver's sensitivity and the driving stability is improved.
In general, by reducing the steering angle ratio at a low velocity the steering amount is reduced and therefore the steering effort is reduced. And, the running stability is secured by increasing the steering angle ratio at a high velocity.
For example, VGR (Variable Gear Ratio) technique, disclosed in Japanese Laid-open Patent No. H07-323852, varies the gear ratio depending upon the steering angle by increasing the mating pitch from the center of rack teeth toward the end of a rack and pinion. Japanese Laid-open Patent No. H11-78937 discloses VGS (Variable Gear-Ratio Steering) technique for varying the gear ratio according to the vehicle velocity and the steering wheel angle.
Japanese Laid-open Patent No. H11-301507 discloses VGRS (Variable Gear Ratio Steering) technique for varying the gear ratio according to the vehicle velocity and the road surface friction factor and adding a differential operation of steering.